This invention relates to lateral support systems for structures subject to dynamic lateral loads. More particularly, it relates to lateral support structures and connections therefor in which selected members are coupled so as to provide selective predetermined substantially frictionless relative motion between the members.
Typically, buildings and other structures are designed primarily to resist gravity loads. That is, the arrangement and sizing of the structural elements is determined by considering only gravity loads. Gravity loads include the weight of the building itself, the weight of attachments to the structure such as pipes, electrical conduits, air-conditioning, heating ducts, lighting fixtures, coverings, roof coverings, and suspended ceilings (i.e., dead load) as well as the weight of human occupants, furniture, movable equipment, vehicles stored goods (i.e., live load).
Structures designed to resist forces caused by dynamic lateral loads such as wind, earthquakes, explosions, vibrating machinery, temperature changes and long-term, gradual distortions due to shrinkage, creep and/or settlement, involve special considerations. Primarily, the principal application of these forces is in a horizontal direction, or, more precisely, in a direction perpendicular (or lateral) to the direction of gravity.
For example, the application of wind force to a closed building create lateral pressures applied normal to the exterior surfaces of the building. These forces may be either inward (i.e., positive pressures) or outward (i.e., negative or suction pressure). The shape of the building and direction the wind determine the distribution of pressures on the various exterior surfaces of the building. The total effect on the building is usually determined by considering the vertical profile, or silhouette, of the building as a single vertical plane surface at right angles to the wind direction.
During an earthquake, the ground surface moves in all directions. The most damaging effect on structures, however, is caused by movements in the direction parallel to the ground surface (i.e., horizontally). Thus, for design purposes, the major effect of an earthquake is usually considered in terms of horizontal force, similar to the effect of wind.
Since most structures are conceived in terms of their gravity resistance, designing for dynamic loads such as winds or earthquakes is often dealt with by bracing the gravity resisting system against lateral forces. In a typical structure, the lateral force system is provided by bracing systems that include solid walls (called "shear walls"), diagonally or otherwise braced bays, and rigid frames. Structures that are designed primarily to resist gravity loads always contain such lateral bracing systems to provide stability against lateral forces induced by unsymmetric load distribution. These lateral bracing systems are usually augmented to provide resistance against lateral forces induced by earthquakes, winds, etc.
The principal concern in structural design for earthquake forces is for the laterally resistant system of the building or structure. In most buildings, this system consists of some combination of horizontally distributing elements (usually roof and floor diaphragms) and vertical bracing elements (shear walls, rigid frames, braced frames, etc.). Failure of any part of this system, or of connections between the parts, can result in major damage to the building, including the possibility of total collapse.
The primary elements of a lateral load resistive system are often braced frames. Post and beam systems, consisting of separate vertical and horizontal members, may be inherently stable for gravity loading, but they must be braced in some manner for lateral loads. The three basic ways of achieving this are through shear panels, moment resistive joints between the members, or by bracing.
When shear panels are used, the panels themselves are usually limited to the direct shear force resistance. Thus, the lateral resistive system is essentially that of a box system, although a complete frame structure exists together with the diaphragm elements of the box.
When moment-resistive joints are used, lateral loads induce bending and shear in the elements of the frame. In rigid frames with moment-resistive connections, both gravity and lateral loads produce interactive moments between the members. In most cases, rigid frames are actually the most flexible of the basic types of lateral resistive systems. This deformation character, together with ductility, make the rigid frame a structure that absorbs energy through deformation.
Most moment-resistive frames consist of either steel or concrete. Steel frames have either welded or bolted connections between the linear members to develop the necessary moment transfers. Frames of concrete achieve moment connections through the monolithic concrete as well as through the continuity and anchorage of the steel reinforcing. Because concrete is basically brittle and not ductile, the ductile character is essentially produced by the ductility of the steel reinforcing.
In braced frames, on the other hand, trussing or triangulation of the frame is used to achieve lateral stability. The trussing is usually achieved by inserting diagonal members in the rectangular bays of the frame. If single diagonals are used, they serve a dual function, acting in tension for the lateral loads in one direction and in compression when the load direction is in the opposite direction. Because tension members are generally more structurally efficient, the frame is sometimes braced with a double set of diagonals (called "X-bracing"). In any event, the trussing causes lateral loads to induce only axial force in the members of the frame, as compared to the behavior of the rigid frame. It also generally results in a frame that is stiffer, having less deformation than the rigid frame.
Significantly, in designing a structure to resist lateral loads, it is not necessary to brace every individual bay of the rectangular frame system. Usually, sufficient bracing is achieved by bracing only a few bays, or even only a single bay. Trussing tends to produce a structure that has a overall stiffness somewhere between that of a stiff diaphragm (shear wall) and that of the flexible moment-resistive frame.
Another major consideration in designing a structure subject to, for example, earthquakes is the detailing of construction connections so that the building is quite literally not shaken apart by earthquake. With regard to the structure, this means that the various separate elements must be positively secured to one another.
According to the prior art, for example, when using trussed structures, it was necessary to ensure that the structure itself is "tight." That is, connections should be made in a manner to assure that they will not be initially free of slack and will not loosen under load reversals or repeated loadings. This meant avoiding connections that are loose or which allow movement between the structural members. Avoiding loose connections is particularly important in systems subject to dynamic loading since relative movement between the structural members leads to increased wear and deterioration of the connection.
As is well known in the art, a zero resistance rotation may be introduced into a structure during the erection of rigidly braced frames. Specifically, certain initial column-girder connections may be constructed to permit rotation at the girder support during the application of a superimposed dead load. After the initial load application, however, a final fixed connection of the columns is installed to prevent free rotation under any additional loading.
Movement in connections or slip response has also been proposed in seismic base isolation systems. In such systems, rigid body motion of the entire structure due to sliding of the foundation provides a constant frictional resistance.
In some other cases it is desirable to allow for some degree of independent motion of selected parts of a structure. In particular, it is desirable to use separation joints to secure various nonstructural elements, such as window glazing, to the structure. These joints permit some degree of independent movement of the nonstructural elements to prevent undesired transfer of force to these elements.
Another type of earthquake resistant system involves "active control". In these systems, a motion sensor detects motion of the structure and activates active controls, such as actuators or other mechanical devices, which counteract the motion. Active control systems are expensive and require maintenance for the electro-mechanical components.